instance (MonadRandom m, MonadTrans t, Monad (t m)) => MonadRandom (t m)

[frasertweedale]

It is common to use monad transformer stacks in applications, with
IO at the heart. For applications using cryptonite, it is useful to
have MonadRandom instances for the common transformers. Add the
instance

instance (MonadRandom m, MonadTrans t, Monad (t m)) => MonadRandom (t m)

which achieves this with little code and one small additional
dependency, 'transformers', with no new transitive dependencies and
upon which many programs are likely to end up depending anyway.
Although new dependencies are discouraged, this change is motivated
by the usefulness of the instance and the desire to avoid orphan
instances in applications.

Also drive-by remove (Functor m) from MonadRandom class constraints,
because Functor is now a superclass of Monad and cryptonite no
longer supports pre-AMP GHC versions.

Fixes: #304

[ocheron]

What if I want:

instance Monad m => MonadRandom (StateT ChaChaDRG m) where
    getRandomBytes n = StateT (return . randomBytesGenerate n)

Lifting is not the only valid implementation.
And an instance for t m would overlap with almost everything.

[ocheron]

The goal of the typeclass is to abstract over different implementations.
We can't force an instance defined for all parameterized types.

[frasertweedale]

@ocheron thanks for the consideration... and a justified rejection.

[ocharles]

What if I want:

instance Monad m => MonadRandom (StateT ChaChaDRG m) where
   getRandomBytes n = StateT (return . randomBytesGenerate n)

AFAICT, this pull request does not stop you doing that. You simply need to mark your instance as overlapping. (Though typically you mark the general instance as overlappable, but it's the same idea).

[ocheron]

Yes but it's not terribly good to rely on instance resolution when the impact of a change is so easily unnoticed.